The present invention relates to a method and apparatus for measuring germicidal (254 nm) ultraviolet radiation (GUVR) in an omnidirectional manner. More particularly the invention relates to those situations in which GUVR from more than one direction or source impinges on a three-dimensional object, and when it is of interest to determine the radiant fluence experienced by the object. In greater particularity the present invention employs spherical UV transmissive vessels constructed of varying outside diameter and filled with an actinometric solution. More particularly, the present invention relates to the use of spherical actinometers which may be read in situ or in a lab environment.
Upper room germicidal (254 nm) UV radiation (GUVR) air disinfection is becoming more widely used for the control of infectious diseases caused by airborne microorganisms such as tuberculosis. Ultraviolet air disinfection has been proposed for use in healthcare facilities and where persons with unrecognized active disease may be present. Although epidemiological studies have shown that UV air disinfection can reduce disease transmission, measurements of UV air disinfection in real-life settings are limited. Low-pressure mercury-vapor lamps used for germicidal application emit approximately 85% of their radiant flux at 254 nm, which is part of the UVC spectrum. The optimal wavelength for air disinfection ranges from 250 to 270 nm, depending upon the microorganism. These lamps are placed near the ceiling and arranged 5-10 feet apart such that the upper room volume is subjected to the germicidal action of the lamp outputs. Baffles and louvers are employed to minimize the amount of GUVR present in the lower portion of the room because overexposure can cause skin and eye irritation. The degree of killing in the room air is a function of the average energy distribution of the GUVR in the upper portion of the room with the provision that exchange of air between the upper and lower portions of the room readily occurs.
For a given arrangement of lamps, there is a need to measure accurately the distribution of GUVR in the upper portion of the room where killing occurs. Normally, for radiation from a sole source, an electronic meter or radiometer is used to measure the irradiance (power delivered per unit area) at the surface of the target, which is assumed to be a planar surface. The irradiance from a given source varies with the cosine of the angle of incidence and radiometer detectors are designed to take this angular dependence into account. However, for purposes of estimating the amount of energy available for killing microorganisms, where the irradiation is from several sources (omni-directional) and the target is three dimensional, the use of irradiance is no longer an option. Instead, the metric of fluence rate must be used, that is the omni-directional radiant flux passing through the cross-sectional area of a small sphere located at some point in space. Such a measurement requires a detector with spherical rather than planar geometry in order to match the geometry of the irradiated object. In this case, energy from all directions is measured equally, the cross-sectional area of the sphere taken as the area of exposure. For the case of irradiation from a single source, located distant from the target such that the radiation is perpendicular to the surface, the fluence rate and the irradiance are the same, the units being watts per unit area. The measurement of fluence rate with a cosine-corrected detector is a conceptual impossibility for anything but collimated (i.e. parallel) radiation at normal incidence.
This problem of measuring radiation in a spherical or non-labertian manner has been addressed previously by Middleton who designed a spherical integrator or illuminometer for operation in the visible portion of the spectrum. More recently, Cabaj and Sommer described the use of a spherical quartz vessel containing Bacillus subtilis spores to measure either UVC or solar radiation. The vessel used was much larger than that described here and was not satisfactory for measuring UVC unless the suspension of spores was well mixed during irradiation.
It is an object of the invention to provide a method for simultaneously measuring the UV fluence at a number of points in space subjected to the combined output of an array of germicidal lamps positioned so as to disinfect the upper room air. Such spherical or 4p measurements provide a more accurate estimate of the energy to which a three-dimensional object such as a microorganism would be exposed in contrast to the energy estimates obtained using a meter or radiometer.
It is another object of the present invention to provide a readily usable spherical actinometer for use in an omnidirectional environment and a methodology for the use of the same. To accomplish these objects the present invention utilizes a device that integrates the omnidirectional radiant flux over time to obtain the radiant energy or fluence incident on the sphere. This spherical actinometer consists of a small spherical vessel containing a solution that responds to germicidal radiation but not ambient room light. In one embodiment the actinometric solution, is an aqueous mixture of iodide and iodate that is optically opaque at 254 nm but insensitive to radiation above 330 nm. The UV-induced formation of triiodide,
I+hxcexd greater than I++e31 xe2x80x83xe2x80x83(1)
xe2x80x83I++exe2x88x92 greater than Ixe2x88x92xe2x80x83xe2x80x83(1a)
2I++Ixe2x88x92=I3xe2x88x92xe2x80x83xe2x80x83(2)
is facilitated by the presence of iodate that acts as an electron acceptor and prevents the back reaction shown in Eq. 1a. The formation of triiodide (Eq. 2), which is easily measured spectroscopically with a photometer described herein, occurs with a quantum yield of 0.75 for 254 nm radiation at 21xc2x0 C.
Another object of the invention is to provide a device for measurement of UV fluence which is substantially independent of the size of the measuring device. This is accomplished in the instant invention by measuring directly in the spherical vessel forming the actinometer due to the relationship of the concentration of the photoproduct to the cross-sectional area of the sphere divided by the volume or 1/r and to the fact that absorbance varies directly in proportion to r.